The invention relates to "lockout" valves which have been used in the mobile hydraulics industry for decades. The only purpose for lockout valves is to eliminate leakage within the hydraulic circuit when the circuit is holding up a static load. A "lockout" is basically a zero-leakage check valve placed in the return line of a system which blocks the return flow from a loaded cylinder to tank except when held open. An example of a "lockout" valve is shown in U.S. Pat. No. 3,613,508, wherein one "lockout" is located in each motor port of a typical hydraulic circuit.
The control and direction of oil in hydraulic circuits is basically controlled by spool-type directional control valves which comprise a slidable spool in a valve body which has various grooves and lands in the spool and corresponding cavities in the valve body to direct and drain the oil to various functions. Because of the relatively loose tolerance in spool valves required to allow them to slide in their bores without binding, the amount of leakage across the spool becomes unacceptable in certain applications. For example, in a circuit which elevates a crane boom, it is unacceptable to have the boom under load in a static condition to move downward or settle due to leakage across the directional control valve spool.
A typical lockout under load is held shut by the force of the load and therefore requires a separate actuating force greater than the load to open the lockout and allow the system to move. Typically this actuating force is provided by pressure in the opposite motor port which actuates a pin to force the lockout open and allow flow to commence in the circuit. The necessity of working pressure to hold the lockout open during operation has limited the application of lockouts to single-acting cylinders, lightly loaded double-acting cylinders or severely orificed double-acting functions. What typically happens when lockouts malfunction is that the pressure necessary in the opposite motor port to hold the lockout open is momentarily lost, due to the system's pump being unable to keep up with the movement in the cylinder under heavy load, thereby causing the lockout to momentarily close due to loss of pressure on the opposite side of the circuit causing the lockout to chatter due to its rapid opening and closing which takes place each time the pump catches up with the load.